The Need for a New Theory of Space and Time - Dynamics and Relativity - page 113

that this statement holds independently of the motion of the source or the observer.
It constitutes a clean break with classical thinking and it is the source of all of the
weird and wonderful physics we shall soon be encountering. As we shall see, it
can ony be true if we reject the notion of absolute time.
To illustrate how weird the 2nd postulate is consider the situation illustrated in
Figure 5.6. A light source sits at rest in S whilst a second frame S moves towards
the light source with a speed v . The 2nd postulate implies that:

An observer in S measures the light to travel towards an observer in S at
a speed c

•

+

v . (When we say “an observer in S ” we shall always mean an
observer at rest in S .)

•

An observer in S measures the light to travel at speed c .

An observer in S

• measures the light to travel at speed c .

The first two of these statements are reasonable but the third really does appear
to be outrageous. Nevertheless it is a necessary consequence of the 2nd postulate.

z

S

z ′

S ′

u

light source

y y ′

O x O ′ x ′

Figure 5.6 A light source sits at rest in S . A second frame S moves towards the light
source with a speed v .

PROBLEMS 5

5.1 A moving walkway moves at a speed of 0.7 ms − 1 relative to the ground and is
20.0 m long. If a passenger steps on at one end and walks at 1.3 ms − 1 relative
to the walkway, how much time does she require to reach the opposite end
if she walks (a) in the same direction as the walkway is moving? (b) in the
opposite direction?
5.2 Convince yourself that the first of the three bulleted items listed at the end
of Section 5.3 is correct.